Injection devices are used for the convenient administration of medicaments. For example, injection devices (which may typically be in the form of a pen injector) may be used for providing a single metered dose of a medicament, for example such as Epinephrine in an emergency or for providing regular metered doses of a medicament, such as Insulin. Such devices may be either single use “disposable” devices in which the device is typically provided with a syringe already installed, and which is not user-replaceable, or “reusable” devices which allow the user to replace the syringe when the medicament has been used.
It may be noted that whilst the term “syringe” is used herein for clarity and consistency, this term is not intended to be limiting. In some arrangements the syringe may for example be a cartridge (which, for example, may be arranged to receive a disposable needle) or other medicament container. In some arrangements the syringe/cartridge/medicament container may formed integrally with the (or part of the) injection device.
Injection devices may be provided in the form of an “autoinjector” device, in which, in addition to automating the delivery of the medicament, the device is also arranged to automate the insertion of a needle into the skin prior to the delivery of the medicament.
Injection devices generally comprise a delivery arrangement which is arranged to automatically deliver a dose from the syringe, and optionally (in the case of an autoinjector) to first displace the syringe within the housing to cause needle penetration. The delivery arrangement generally acts via a plunger which includes or engages a piston (also referred to as a “bung”) which is slidably provided within the syringe. In the case of an autoinjector the initial stiction between the bung and syringe may resist forward movement of the piston relative to the syringe such that initially the delivery arrangement moves the syringe into the needle insertion position (whereupon further movement of the syringe is blocked and the delivery arrangement will continue to move forward thus moving the bung). A common form of delivery arrangement includes an actuation mechanism which biases the plunger forwardly and a trigger mechanism which holds the plunger (directly or indirectly) against the force of the actuation mechanism until the trigger is released. For example the actuation mechanism may comprise a drive spring (for example a compression spring) which is held in an energised (or primed position) prior to release by the trigger.
It will be appreciated that the force required to be provided by the actuation mechanism may depend upon a number of factors and these must be taken into account when designing the injector device. For example the force may depend upon the volume of medicament to be delivered, the viscosity of the medicament and/or the bore of the delivery needle. Whilst some combinations of medicament and syringe/needle size may require relatively high delivery forces the designer must also provide consideration for the reaction of the forces, particularly through the syringe or cartridge body (which for medicament compatibility reasons may be required to be constructed of glass). For example, embodiments of the invention may be particularly useful for use in devices in which a spring force of 30N or more is utilised. Such problems may be especially relevant to autoinjector type devices where it is necessary to arrest the forward movement of the syringe when the needle reaches the required insertion depth. Thus, the applicants have recognised that in some injection devices there may be a risk that the actuation mechanism is required to impart sufficiently high force to the plunger that there is a risk that the initial impact speed between the plunger and bung or between the syringe and the housing may be unacceptably high (for example resulting in an increases risk of damage to the syringe).
At least some embodiments of the invention seek to provide an improved injection device which may help to address some of these problems.